Electrical contact material

ABSTRACT

ELECTRICAL CONTACT MATERIAL CONSISTING ESSENTIALLY OF SILVER AND CADMIUM OXIDE IS DISCLOSED WHEREIN THE MATERIAL IS CHARACTERIZED BY HAVING A PRE-OXIDIZED MICROSTRUCTURE WHEREIN THERE IS A FINE CADMIUM OXIDE PARTICLE SIZE AND A SUBSTANTIALLY UNIFORM DISTRIBUTION OF CADMIUM OXIDE THROUGHOUT THE MATERIAL INCLUDING THE CORE. THE MATERIAL HAS A LOWER ARC EROSION RATE THAN POST OXIDIZED MATERIAL OF THE SAME COMPOSITION WHEN TESTED UNDER SIMILAR CONDITIONS. A BACKING, FOR EXAMPLE OF FINE SILVER MAY BE PROVIDED FOR THE CONTACT MATERIAL.

July 4, 1972 R HAARBYE EI'AL 3,674,445

ELECTRICAL CONTACT MATERIAL Original Filed Dec. 9, 1966 2. Sheets-Sheet 1 m 0 400 Z 3 0 CL 1 8 a: 300 O u.

. J m Lu :1.

PRE-HEAT TEMPERATURE C INVENTOR' 5 FREDRIK O. HAARBYE BY M w. M

ATTOR NEY 2 Sheets-Sheet 2.

F. O. HAARBYE ETAL ELECTRICAL CONTACT MATERIAL July 4, 1972 Original Filed Dec. 9, 1966 6 RE 4 2 OY 2 TB NR 4 A Y 2 2 W E 2 1H m 0 2 T S K A m m m m N D NW K E 8 m R Y 6 E l W F B l T U 4 W .w W 4 m I 2 D 6 E E T F 8 E E m C a m 6 P 6 4 2 O O O m 0 m 0. w w W D O 4 4 3 2 2 O O O 0 O O O O 5 O 5 O 5 O 5 O 8230" Lo o. 5; 4 4 3 3 2 2 n mczDonTmomom Jmwm Patented July 4, 1972 3,674,446 ELECTRICAL CONTACT MATERIAL Fredrik 0. Haarbye and Peter C. Murphy, Indianapolis,

Ind., assignors to P. R. Mallory & (10., Inc., Indianapolis, Ind.

Application Dec. 9, 1966, Ser. No. 600,621, now Patent No. 3,545,067, which is a continuation-impart of application Ser. No. 445,038, new Reissue Patent No. 27,075. Divided and this application Oct. 13, 1969, Ser. No. 870,837

Int. Cl. B32b /04; H01h 1/02 US. Cl. 29-195 22 Claims ABSTRACT OF THE DISCLOSURE Electrical contact material consisting essentially of silver and cadmium oxide is disclosed wherein the material is characterized by having a pro-oxidized microstructure wherein there is a fine cadmium oxide particle size and a substantially uniform distribution of cadmium oxide throughout the material including the core. The material has a lower arc erosion rate than post oxidized material of the same composition when tested under similar conditions. A backing, for example of fine silver may be provided for the contact material.

This application is a division of application Ser. No. 600,621, filed Dec. 9, 1966, now US. Pat. 3,545,067 which is a continuation-in-part of application Scr. No. 445,038, now US. Reissue Pat. 27,075 which is incorporated into the present application by reference.

Silver-cadmium-oxide strips for use as electrical contact materials are being increasingly used in many electrical applications. For the most part this electrical contact material is manufactured by two basic processes: (1) strip manufactured by casting bars of silver-cadmium alloy, rolling the bar to approximately the desired strip thickness, internally oxidizing the silver-cadmium strip, and where necessary, following with a rolling operation to size the strip, and (2) manufacture of pre-oxidized strip consisting of stratified layers of fine and coarse cadmium oxide and areas depleted in CdO. These processes have inherent disadvantages. For the most part there will be a non-uniform distribution of cadmium oxide; there will be a depleted center core; there is a thickness limitation of the strip; and control of the particle size is difiicult.

In copending application S.N. 445,038, filed Apr. 2, 1965, in the name of Fredrik O. Haarbye, there is described a process for manufacturing pre-oxidized silvercadmium oxide material from shot grain or pellets. As is more completely described in that application, this process in general comprises the steps of forming a silver-cadmium shot, internally oxidizing the shot, compacting the oxidized shot, extruding the shot, and cold working the extruded shape to final size. This process has succeeded in overcoming to a great extent, the disadvantages of the previously noted processes.

Having once made the silver-cadmium oxide strip, there then arises the problem of attaching the strip to a suitable backing material such as stainless steel, for example, to form the electrical contact. Normally this is done by welding or brazing. However, it has been found that silver-cadmium oxide contact materials are extremely difiicult to attach directly to the stainless steel backing material. Silver-cadmium oxide strips are, therefore, first backed with a layer of fine silver. In the case of wrought silver-cadmium oxide materials (post-oxidized), this fine silver layer is applied to the contact material, the contact material being in ingot or bar form. It has been found, however, that this process cannot be employed in the case of pre-oxidized silver-cadmium oxide materials made from the process of the above-noted copending application.

The present invention is concerned with the provision of a novel electrical contact material consisting of a silver-cadmium oxide material and has as one of its ob jects the provision of such a material that has a more uniform distribution of cadmium oxide.

Another object of the invention is the provision of such a material that has no depletion center core.

Another object of the invention is the provision of such a material wherein there are no stratified layers of fine, coarse, and depleted cadmium oxide particles.

Still another object of the invention is the provision of such a material that has a random distribution of cadmium oxide particle sizes throughout the contact material.

Yet another object of the invention is the provision of such a strip material having a backing layer such as fine silver to form a bi-metal electrical contact material.

Still another object of the invention is to provide a method of applying such layers of fine silver to the silver-cadmium strip material.

Another object of the invention is to provide a hot roll-bonding method of applying a fine silver layer to the silver-cadmium oxide strip.

With the above and other objects in view, which Will appear as the description proceeds, this invention resides in a novel electrical contact material and a process for making the same substantially as described herein and more particularly defined by the appended claims, it being understood that such changes in the precise embodiments here disclosed may be made as come within the scope of the claims.

Inthe drawings:

FIG. 1 is a cross-section of the silver-cadmium oxide strip made in accordance with the present invention.

FIGS. 2 and 2a are cross sections of silver-cadmium oxide strips made by prior art methods, and

FIGS. 3 through 5 are curves showing the efiects of varying certain process variables in fabricating the strip in accordance with the invention.

Generally speaking, the objects of the invention are accomplished by providing a strip of an electrical contact material consisting essentially of silver-cadmium oxide, the cadmium oxide being in an amount up to 30% by weight, the strip material having a microstructure Wherein there is a random distribution of particle sizes of the cadmium oxide throughout the strip with no depleted cores. In one embodiment the material has a backing of, for example, fine silver integrally connected thereto.

The cadmium oxide content is limited to a maximum of 30% by weight because of poor contact characteristics and the inability to process the material by wrought metal processing techniques.

The method of manufacturing the silver-cadmium oxide material with the integrally connected silver backing in its broadest aspect comprises forming, for example by casting a silver-cadmium shot, internally oxidizing the shot, compacting the oxidized shot, extruding the shot, hot-roll bonding the backing to the extruded shape, and then rolling the resulting composite to the desired final thickness.

The microstructure of the electrical contact material of the present invention is more clearly shown with reference to FIG. 1. As shown there is a random distribution of particle sizes of the cadmium oxide 10 throughout the contact material. This is in contrast to other silver-cadmium oxide electrical contact materials wherein, for example, as shown in FIGS. 2 and 2a, there are depleted zones or cores 20 of cadmium oxide. In FIG. 2,

the material has been post-oxidized while in FIG. 2a, the material has been pre-oxidized in stratified layers. Such depletion zones causes the electrical contact material to have a strong tendency to weld or stick together. In addition to the advantage derived from there being no depletion cores, the random particle size distribution throughout the material makes for a more uniform predictable material.

Pre-oxidized silver cadmium oxide strip made in accordance with this invention has been evaluated as an electrical contact material using a general purpose clapper type high current contactor. Post-oxidized material was tested in the same program in order to obtain comparative results.

Contacts /8" in diameter were brazed to studs and assembled in the contactor. They were weighed at the start and finish of the test to obtain weight loss measurements. The voltage drop was measured at the start and end of the test with rated current through the contacts, and the temperature of the stationary contact support was monitored during the test. The contacts were tested for 20,000 operations making and breaking 150 amperes at 240 volts A.C., and 45 %-50% power factor.

The materials tested were:

Ag 1 CdOpre-oxidized Ag+ 15 CdO pre-oxidized Ag+ 10% CdOpost-oxidized Ag+ 15 CdOpost-oxidized As indicated in the following table the eelctrical erosion (weightloss), voltage drop, and temperature rise all were lower on the pre-oxidized materials than on the post-oxidized materials of a comparable composition. The lower values signify superior contact performance.

Wt. loss Maximum per contempera- Average tact pair ture rise millivolt Drop Test No. Material (grams) 0.) start end 3610 Ag-10% OClO 0275 43 28 12 pre-oxidized.

Average d0 0234 42. 3 26 13. 7

3670 Ag-10% CdO 0436 43 23 15. 6

postpxidized.

Average d0 0361 46. 7 27. 7 19. 3

3619 Ag-15% CdO .0375 45 31 17 pre-oxidized.

Average do 0365 3637 Ag-l% CdO .0460

p0st-oxidized. o 0487 0475 Average d0 0473 While not desiring to be so limited, it is felt that the novel grain structure of the silver-cadmium oxide electrical contact material is achieved through the preoxidizing shot extrusion process of the above-noted application S.N. 445,038, due to oxidation of metal particles of small volumes. Since the cadmium oxide particle size is proportional to the total distance of oxygen travel during oxidation, a control of the cadmium. oxide particle size is provided through control of the metal shot or pellet size. By reducing the cadmium oxide particle size, the surface area of the oxide is greatly increased without adversely affecting the electrical conductivity of the contact material. This increased surface area of cadmium oxide is believed beneficial in reducing the welding or sticking tendency of the contact during service. :Also, the small size of the cadmium oxide as shown for example by the hardness and tensile strength curves in- FIG. 6 of said Ser. No. 445,038 (now U.S. Reissue Fat.

27,075) particles has a hardening effect on the contact material, making this contact material less susceptible to wear. A practical method of control of cadmium oxide particle size has not existed prior to the method described in this invention. Also, because the oxygen path during oxidation is relatively short, oxidation is completed before any depletion of cadmium has occurred in the central portion of the metal shot or pellet, and size limitations of this type contact material in strip form are only governed by the mechanical limitations of the processing equipment, and not by metallurgical factors as in the case with post-oxidized material.

The present invention also contemplates providing the silver-cadmium strip with a backing material, such as fine silver, integrally connected thereto to form a bi-metal strip. Such backing strip is necessary in order to bond the silver-cadmium strip to a material such as stainless steel soas to form an electrical contact.

The preferred specific steps for forming the bi-metal strip after the shot has been extruded, in accordance with the process of the aforementioned copending application SN 445,038 are:

(1) Cut material for strip sandwich (2) Tack weld one end of strip sandwich (3) Preheat strip sandwich (4) Roll-bond strip sandwich (5) Roll-clad strip sandwich (6) Roller-level clad strip and knurl silver side (7) Trim back end of clad sandwich (8) Slit clad sandwich to finish width As shown in FIGS. 3 through 5, there are three process variables which have a marked effect on bonding quality between the fine silver backing and the silver-cadmium strip. In the curves shown in the figures the peel strength, or the force necessary to separate the backing from the strip, is used as a parameter of the bond quality. As indicated in FIG. 1, optimum bond quality is obtained by utilizing a preheat temperature of from about 775 C. to 825 C. Such preheating is preferably done in a pullthrough furnace. -At higher temperatures there is danger of melting during preheat as well as danger of sticking of the metal sandwich to the rolls during rolling. At lower temperatures, insufiicient diffusion takes place to give a complete bond over the entire interface. As indicated in FIG. 4, the length of time the metal sandwich is subjected to the preheat is of importance where long lengths are being fabricated. In general, a preheat time of from about 10 to 20 minutes is preferable for the noted pre heat temperature.

The amount of reduction required for good bond strength is important for two reasons: it establishes intimate contact between the mating surfaces and creates additional fresh surface area free of metal oxide or impurities. However, if the reduction is too great, surface cracking and other defects appear. In general, as indicated by the curve of FIG. 5, a minimum reduction of about 1 0% is needed to achieve a minimum backing strength, with an optimum being from about 10 to 25%.

The following illustrates the method for fabricating the novel electrical contact strip of silver-cadmium oxide with a fine silver metal backing.

A molten bath consisting of 70 pounds of silver-10% cadmium oxide by weight was superheated to about 12-60 C. This molten bath was poured into a cast iron funnel having a plurality of apertures in a graphite insert. As the molten metal passed through the apertures it was subjected to an air 50% water stream flowing about normal to the molten metal such that the molten metal stream was broken into small fragments. The metal was then collected and cooled to a temperature of about 70 C. to form a silver cadmium shot having a shot size about mesh. The shot was then air dryed at room temperature and then screened to a size of about mesh. The shot was then oxidized by heating in an air atmosphere at 775- 815 C. for 24 hours. The oxidized shot was then compacted under a pressure of about 85/90 t.s.i. and then extruded to form a strip .500" x .l25" x 50'.

The extruded strip was then cut to a length of about 3' and a fine silver strip of about .015" thickness was tack welded to it. The resulting sandwich was then preheated in a batch furnace at a temperature of about 800 C. for 15 minutes. The fine silver backing was then roll-bonded to the silver-cadmium strip to form an integral part by passing the sandwich through a pair of rolls spaced .125 apart. The resulting bi-metal strip was then reduced to a thickness of about .100", a reduction of about 23% and roller leveled, the reduction being made in a single pass.

From the foregoing description taken in conjunction with the drawings it will be apparent to those skilled in the art that this invention provides a new and improved electrical contact material and a method of making the same. Accordingly, it is contemplated that the scope of the invention is to be determined from the claims appended hereto.

What is claimed is:

1. Electrical contact material consisting essentially of silver and cadmium oxide, said material being characterized by having a pre-oxidized microstructure wherein there is a fine cadmium oxide particle size and a substantially uniform distribution of cadmium oxide throughout the material including the core, said material having a lower arc erosion rate than post oxidized material of the same composition when tested under similar conditions.

2. Electrical contact material according to claim 1 wherein the cadmium oxide content is not above about 30% by weight.

3. Electrical contact material according to claim 1 wherein the test conditions include an A.C. current of about 150 amperes.

4. Electrical contact material according to claim 2 wherein the tests are carried out for at least 20,000 operations.

5. Electrical contact material according to claim 4 wherein the cadmium oxide is at least about 10% by weight.

6. Electrical contact material according to claim 5 wherein the test conditions include a voltage of about 240 volts A.C. and a 45% to 50% power factor.

7. Electrical contact material according to claim 5 wherein the tests are carried out for 20,000 operations.

8. Electrical contact material according to claim 6 also having an annealed hardness above that obtained with post-oxidized material when hardness tested under similar conditions.

9. Electrical contact material according to claim 5 having a backing member integrally bonded thereo.

10. Electrical contact material according to claim 9 wherein said backing material is silver.

11. Electrical contact material consisting essentially of silver and cadmium oxide in the form of strip said material being characterized by having a pre-oxidized microstructure wherein there is a fine particle size and a substantially uniform distribution of cadmium oxide throughout the material including the core, said material having a lower arc erosion rate than post oxidized material of the same composition when tested under similar conditions.

12. Electrical contact material according to claim 11 wherein the cadmium oxide contact content is not above about 30% by weight.

13. Electrical contact material according to claim 10 wherein the test conditions include an A.C. curent of about amperes.

14. Electrical contact material according to claim 12 wherein the tests are carried out for at least 20,000 operations.

15. Electrical contact material according to claim 14 wherein the cadmium oxide is at least about 10% by weight.

16. Electrical contact material according to claim 15 wherein test conditions include a voltage of about 240 volts A.C. and a 45% to 50% power factor.

17. Electrical contact material according to claim 16 wherein the tests are carried out for 20,000 operations.

18. Electrical contact material according to claim 16 also having an annealed hardness above that obtained with post oxidized material when hardness tested under similar conditions.

19. Electrical contact material according to claim 12 having a backing member integrally bonded thereto.

20. Electrical contact material according to claim 10 wherein said backing material is silver.

21. 'Electrical contact material according to claim 2 having a backing member integrally bonded thereto.

22. Electrical contact material according to claim 21 wherein said backing material is silver.

References Cited UNITED STATES PATENTS 2,539,298 1/1951 Doty et a1 29-199 2,673,167 3/1954 Vines 14832 2,221,286 11/1940 Hensel et al. 200l6 6 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 

